NEW YORK -- A ceremony honouring six people who died 20 years ago in the first terrorist attack on the World Trade Center was being held Tuesday at the 9-11 memorial, where the twin towers were destroyed eight years later.

More than 1,000 people were injured in the 1993 blast in an underground garage below one of the towers. It was the first dramatic demonstration that "terrorism is theatre and New York is the biggest stage," Police Commissioner Raymond Kelly said last week.

Six Islamic extremists were convicted of carrying out the bombing, including mastermind Ramzi Yousef. The 2001 attack brought down the World Trade Center, where more than 2,700 people died.

The ceremony was being held at the memorial, and a moment of silence will be observed at 12:18 p.m. EST (1718 GMT), the time when a truck bomb was detonated below the north tower.

At first, officials assumed the explosion on a chilly day was an accident. The initial report to police that day called it an apparent transformer explosion at the trade centre.

Kelly raced to the scene, where the bomb planted in a parked Ryder van had left a crater half the size of a football field in the trade centre garage, causing more than a half-billion dollars in damage.

"I remember seeing this tremendous sea of first-responder vehicles ... and smoke was coming out," said Kelly, who was on his first stint as police commissioner.

The trade centre stood in the darkness that night for the first time since it opened in 1973.

It was only the next day, after a utility mishap was ruled out, that authorities "started to come to the conclusion it was bomb," Kelly said.

Investigators then found a vehicle identification number on a piece of the blown-up van that they traced to Mohammed Salameh, who had rented the van.